Improve cancellations, randomness, and time

- Exhaustively document cancellation points
- Rename SIGCANCEL to SIGTHR just like BSDs
- Further improve POSIX thread cancellations
- Ensure asynchronous cancellations work correctly
- Elevate the quality of getrandom() and getentropy()
- Make futexes cancel correctly on OpenBSD 6.x and 7.x
- Add reboot.com and shutdown.com to examples directory
- Remove underscore prefix from awesome timespec_*() APIs
- Create assertions that help verify our cancellation points
- Remove bad timespec APIs (cmp generalizes eq/ne/gt/gte/lt/lte)

libc/calls/clock_nanosleep-openbsd.c

@@ -31,8 +31,8 @@ int sys_clock_nanosleep_openbsd(int clock, int flags,
       res = sys_nanosleep(req, rem);
     } else {
       sys_clock_gettime(clock, &now);
-      if (_timespec_gt(*req, now)) {
-        rel = _timespec_sub(*req, now);
+      if (timespec_cmp(*req, now) > 0) {
+        rel = timespec_sub(*req, now);
         res = sys_nanosleep(&rel, 0);
       } else {
         res = 0;